This invention relates to a process for the purification of aqueous solutions polluted by nitrate ions.
The invention also relates to recycling of the solid compounds emanating from the above-mentioned purification process, preferably in the field of fertilizers when the bound nitrate content is sufficient.
Nitric acid is one the most widely used mineral acids. Nitric acid is used on a very large scale in the chemical industry, predominantly for the manufacture of artificial fertilizers, particular in the form of ammonium nitrate. Nitric acid, in addition, is of very great importance in the synthesis of many explosives.
The salts of nitric acid, nitrates, are also well known and widely used in the chemical and metallurgical industries.
The use of nitric acid or its salts is accompanied, in most cases, by the discharge of effluents containing nitrate ions. Industrial facilities also discharges various other forms of nitrogen which may be converted into nitrates: ammoniacal nitrogen is discharged, in particular by coking plants, and nitrogenous organic compounds are discharged, in particular, by the agri-foodstuffs plants.
Nitrates can have a natural origin, such as in the form of deposits of salts, generally of sodium nitrate.
Nitrates can also emanate from the conversion of ammoniacal nitrogen by biochemical nitrification. Liquid manure constitutes one of the raw materials richest in ammoniacal nitrogen: approximately half the nitrogen present in liquid manure is in organic form, and approximately half in ammoniacal form (that is on the order of 2 to 3.5 g/l of NH.sub.4.sup.+). In smaller proportions, municipal waste water also contains ammoniacal nitrogen which can likewise be converted into nitrates biochemically.
To accomplish nitrification biochemically, it can be sufficient to aerate the liquid manure when the latter naturally contains the appropriate bacteria. The nitrogen present in liquid manure is modified to only slightly during storage of the liquid manure; at the very most, a slight loss takes place through outgassing of ammonia. Spreading on the ground causes mineralization of the readily decomposable organic matter and nitrification of NH.sub.4.sup.+. The nitrates produced may be used by plants, assimilated by microorganisms or leached in depth into the soil beyond the root absorption zone; they are then lost to the plant and contribute, to long or short term pollution of groundwater.
Whether of synthetic or natural origin and irrespective of the chemical nature of the counter anion, nitrates are, for the most part, soluble in aqueous media in high proportions. As an example, solubility in water of calcium nitrate, Ca(NO.sub.3).sub.2, reaches 1.212 kg per liter at 18.degree. C. and that of sodium nitrate, NaNO.sub.3, reaches 0.921 kg per liter at 25.degree. C.
The presence of ever increasing amounts of nitrates in the natural environment constitutes a matter growing of concern. It is known that, taken in repeated small doses, nitrates are potentially toxic due to the possible conversion of nitrates to nitrates, which can cause fatal methemoglobinemia in young children, and to nitrosamines, which are reputedly carcinogenic. For this reason, doctors recommend giving only water free from nitrate ions to young children. In point of fact, as a result of their high solubility in aqueous media, the concentration of nitrates, whether they originate from discharges of industrial effluents, from municipal waste water or from chemical or natural fertilizers (spreading of liquid manure), is gradually increasing in surface water and groundwater which, in some instances, has become unfit for human consumption.
The nitrate concentration is limited to 50 mg per liter in drinking water in France and in the European Community in general. However, as a result of the lack of a purification plant suited to the removal of nitrates on French territory, nitrate ion concentrations which can range up to 100 mg per liter can be found in some drinking water supplied to the water.
To solve this problem, various purification processes are currently in use for the treatment of water intended for human consumption. However, no economically advantageous process that can be implemented on an industrial scale exists for the treatment of effluents highly polluted with nitrates, such as liquid manure.
One type of process used for obtaining drinking water by physicochemical means involves the removal of nitrate ions by denitration with an ion exchange resin. This process requires water possessing a content of suspended solid matter of less than 1 mg per liter; otherwise, the water to be treated must first be subjected to sieving, followed by one or more flocculation, filtration and/or settling, and physical separation, operations.
The ion exchange resin retains the nitrate ions, as well as sulfate ions, chloride ions and hydrogencarbonate ions.
A main drawback of this process is that it cannot be used if there is an excessive amount of ions and chloride ions in the water, which have affinities which are, respectively, much greater than and similar to that of nitrates for the resins. Since regeneration of the resins is generally carried out with sodium chloride, a replacement of the nitrate ions by chloride ions takes place during the purification process. This results in an increase in the chloride ion content in the water treated by this process. Another major drawback of this process involves the disposal of the eluates which are rich in chlorides, nitrates, and often sulfates, obtained after regeneration of the resins. If the concentrations are not too high, they go into the nearest stream. In the other cases, they are either sent to the nearest biological purification plant or stored.
In another type of biological process, nitrate ions are removed using bound bacteria capable of metabolizing these ions. Usually, heterotrophic bacteria drawing their energy from a carbonaceous nutrient, such as ethanol or acetic acid, are used. By this process, the nitrate ions are converted into gaseous nitrogen. Proliferation of the bacteria leads to an excess of biomass or sludge, which can be treated with municipal sludges or incinerated.
This process can be used only at temperatures above 8.degree.-10.degree. C. In addition, these temperatures must be stable during the treatment; otherwise, control of the process is complex. This process is consequently relatively expensive to perform.